Nacreous pigments, also known as pearlescent or effect pigments, exhibit pearl-like and/or iridescent effects upon the transmission and reflection of light therethrough. As is well known in the art, the characteristics of such pigments depends upon optical interference phenomena as more fully described, for example, in xe2x80x9cThe Properties of Nacreous Pigmentsxe2x80x9d, Greenstein and Miller, Technical Papers, Vol. XIII, Annual Technical Conference, Society of Plastic Engineers, May 1967.
Numerous patents and publications have described effect pigments based on titanium dioxide coatings on various substrates. Early examples include Linton U.S. Pat. No. 3,087,828 and 3,087,829 which describe the preparation of titanium dioxide and other metal oxide coated mica effect pigments, which optionally can be topped with a layer of another material such as, inter alia, iron. Since that time, numerous patents and publications have described the overcoating of titanium dioxide-coated mica to form a variety of effect pigments.
Recently, there has been renewed interest in a type of effect pigments known as xe2x80x9coptically variablexe2x80x9d because they exhibit different colors at different viewing angles, i.e., they exhibit color travel or xe2x80x9cflopxe2x80x9d as the angle of viewing changes. Such optically variable pigments have been described in the patent literature since the 1960s. For instance, Hanke in U.S. Pat. No. 3,438,796 describes the pigment as being xe2x80x9cthin, adherent, translucent, light transmitting films or layers of metallic aluminum, each separated by a thin, translucent film of silica, which are successively deposited under controlled conditions in controlled, selective thicknesses on central aluminum film or substratexe2x80x9d. These materials are recognized as providing unique color travel and decorative optical color effects.
The recent approaches to optically variable pigments have generally adopted one of two techniques, both of which are designed to position a low refractive index layer such as silica (Rf 1.5) between reflective layer. In the first, a stack of layers is provided on a temporary substrate which is often a flexible web. The layers are generally made up of aluminum, chromium, magnesium fluoride and silicon dioxide. The stack of film is separated from the substrate and subdivided into appropriately dimensioned flakes. The pigments are produced by physical techniques such as physical vapor deposition onto the substrate, separation from the substrate and subsequent comminution or by other deposition techniques (plasma, sputtering etc.), subsequent deflaking of the decomposition product, etc. In the pigments obtained in this way, the central layer and all other layers in the stack are not completely enclosed by the other layers. The layered structure is visible at the faces formed by the process of comminution.
In the other approach, a platelet shaped opaque metallic substrate is coated or encapsulated with successive layers of selectively absorbing metal oxides and non-selectively absorbing layers of carbon, metal sulfide, metal and/or metal oxide. To obtain satisfactory materials using this approach, the layers are applied by multiple techniques such as chemical vapor deposition and/or sol-gel processes. A major shortcoming of this is that traditional metal flakes usually have structural integrity problems, hydrogen outgassing problems and other pyrophoric concerns.
The prior art approaches suffer from additional disadvantages. For instance, certain metals or metal flakes such as chromium, aluminum, copper, brass and bronze may have perceived health and environmental impacts associated with their use.
New optically variable effect pigment which do not suffer from the disadvantages of the prior art are clearly desirable and it is the object of the present invention to provide the same.
This invention is related to new articles exhibiting optically variable color and high reflectivity and their preparation. More particularly, the invention relates to a platelet pearlescent pigment having an oxide coating on titanium dioxide-coated substrate platelets from which a portion of the substrate has been eliminated.
The optically variable effect pigments of the present invention are oxide-coated, for instance, iron oxide-coated, titanium dioxide platelet pigments from which a portion of the interior of the titanium dioxide has been removed. These titanium dioxide platelet pigments are derived from titanium dioxide-coated siliceous (e.g., mica) substrates from which a portion of the substrate has been removed. The formulation of coating and other compositions containing the resulting pigments and the coating of substrates is known.
Appropriately sized titanium dioxide platelets commonly referred to as xe2x80x9cplaty TiO2xe2x80x9d or xe2x80x9cself supporting TiO2xe2x80x9d are described, for instance, in U. S. Pat. No. 4,192,691 and 5,611,851. Such platelets are substantially substrate free, generally containing less than about 20% of substrate based on the total weight of the product. U.S. Pat. No. 4,192,691 employs an aqueous solution of hydrofluoric acid and a mineral acid such as sulfuric acid to dissolve the mica from the pigment. It also discloses and illustrates the use of this dissolving agent to remove the mica from a titanium dioxide-coated mica having a surface layer of either iron or chromium oxide. U.S. Pat. No. 5,611,851 employs a combination of a mineral acid and phosphoric acid followed by an extractive dissolution using an alkali. Although the procedure of U.S. Pat. No. 5,611,851 is preferred, other procedures can be employed to obtain the titanium dioxide platelets used in the present invention. Titanium dioxide platelet types suitable for use in this invention can be prepared by removing gypsum from TiO2 coated gypsum or by burning off graphite from TiO2 coated graphite. Dissolving glass from a TiO2 coated glass base also provides a substrate useful in this invention. Although there are several avenues for preparing the TiO2 platelets which then can be coated further, the TiO2 substrate of U.S. Pat. No. 5,611,851 is still preferred in order to obtain maximum reflectivity and color purity. Initially using a substrate aids in producing the relatively smooth and regular titanium dioxide surfaces needed to achieve high quality effect pigments, and the subsequent removal of the mica (refractive index 1.5) or other substrate and its replacement with air (Rf 1.0), allows the benefit of the refractive index of TiO2 (2.6-2.9) to be more fully realized.
The platy TiO2 pigments exhibit little, if any, color travel and cannot be considered optically variable. Surprisingly, however, it was discovered that when the average particle size (longest dimension measured by SEM) was limited, oxide overcoating produced a sharp and distinct color travel.
Accordingly, the platelets of titanium dioxide used in the present invention generally have an average longest dimension of about 1-25 xcexcm, preferably about 2-15 xcexcm and more preferably about 5-8 xcexcm. The platelets can have a thickness of about 5-600 nm, and is more preferably about 20-400 nm. The TiO2 is preferably in the rutile crystalline form but can also be in the anatase form.
While the use of platelets which are substantially substrate free, i.e., generally containing less than about 20% of substrate based on the total weight of the product, provides optically variable effect pigments, the need to eliminate so much of the substrate adds to the manufacturing cost. Also, because the center of the platy TiO2 is essentially hollow, the pigment tends to be more fragile which, in turn, tends to complicate its use in applications where the pigment is subjected to more rigorous conditions. However, it has been found that the titanium dioxide need not be substantially substrate free, and platelets where only a part of the core substrate has been eliminated gives similar results while reducing manufacturing cost and making the pigment more sturdy, allowing it to be employed in more rigorous applications such as in automotive paints.
Accordingly, the platelets used in the present invention are at least partially core (substrate) free, in that at least about 30% of the substrate core has been removed. The platelets may be substantially substrate free but are preferably partly core-free. By partly core-free is meant that about 30 to 60% of the mica or other siliceous substrate in the platelet has been removed. When no substrate has been removed, its content is dependent on the thickness of the TiO2 and generally constitutes about 40-90%, and more often about 60-80%, of the total weight. The amount of substrate after removal based on the total weight of the platelet will thus always be less than about 63%, and more usually less than about 56%. In the case of the preferred partly core-free platelets, the substrate is most usually about 24-50% based on the total weight of the platelet.
The intermediate product produced in the process disclosed in U.S. Pat. No. 5,611,851, i.e., the partly core-free platelet product realized before the alkali extractive dissolution, is very useful in the present invention and is preferred. In this procedure, any TiO2-coated substrate effect pigments known heretofore is subjected to an acid extractant which is a combination of phosphoric acid and one or more mineral acids such as sulfuric acid, hydrochloric acid and nitric acid. In general, the acid solution can contain up to about 20% of the phosphoric acid, for instance about 1%-20%, preferably about 10%-15%, and up to about 35% of the mineral acid, for instance 5-35% and preferably about 25%-30%. The ratio of mineral acid to phosphoric acid can vary over a wide range of from 10:1 to 1:10 but preferably the mineral acid is present in excess such that the ratio is greater than about 1:1 up to about 3:1. In the case of a mica substrate, the extractive dissolution is continued until the desired degree of aluminum and potassium components of the mica have been removed which can take as short a period of time as xc2xc of an hour to as long as 20 hours or more, preferably about 4 to 8 hours. Preferably, substantially all of the Al and K is removed, i.e., possibly only trace amounts remain. The extractive dissolution can be carried out at any convenient temperature such as those from about 20 to 150xc2x0 C. As a general rule, the higher the temperature, the faster the dissolution. Preferably, the extractive dissolution takes place at reflux. Before being overcoated, the partly core-free platelets can be separated from the reaction mixture in which they were prepared in any convenient fashion, such as by hot or cold filtering, and then washed and dried.
The methods used to provide the at least partially core-depleted titanium dioxide-coated substrate platelets with various surface layers, such as iron oxide or other oxides having a refractive index of about 1.5-2.5 (Si, Sn, Cr, etc.), is well known. In broad terms, the material to be coated is brought into contact with a salt of the metal, usually an aqueous solution thereof, under appropriate conditions, e.g. pH, so as to deposit a layer of hydrous metal, followed by calcination which results in formation of oxide(s). The present invention utilizes such known procedures but differs therefrom with respect to the titanium dioxide platelet used as the material to be coated. In general, the oxide overcoating will constitute about 4-20%, preferably about 4-10%, of the final calcined product.
The overcoated oxide may be inherently colorless such as alumina, zirconium oxide, zinc oxide, tin oxide, antimony oxide or even an additional layer of titanium dioxide, or may be inherently colored such as iron oxide, nickel oxide, cobalt oxide, copper oxide or chromium oxide, or may be a mixture of various oxides. Further overcoatings also known in the art can be present.
Compared to prior art, the process of the present invention has several advantages. It allows color variable effect pigments to be made employing standard, pearlescent coating technology without resorting to organic solvent based reactions or chemical vapor deposition/reduction techniques, it takes full advantage of the high refractive index of TiO2 (2.6-2.9), it does not require costly or impractical equipment, and it produces a full range of colors which can be used in cosmetic/automotive/industrial markets.